Baseball and softball bat manufacturers are continually attempting to develop ball bats that exhibit increased durability and improved performance characteristics. Ball bats typically include a handle, a barrel, and a tapered section joining the handle to the barrel. The outer shell of these bats is generally formed from aluminum or another suitable metal, and/or one or more composite materials.
Barrel construction is particularly important in modern bat design. Barrels having a single-wall construction, and more recently, a multi-wall construction, have been developed. Modern ball bats typically include a hollow interior, such that the bats are relatively lightweight and allow a ball player to generate substantial “bat speed” or “swing speed.”
Single-wall bats generally include a single tubular spring in the barrel section. Multi-wall barrels typically include two or more tubular springs, or similar structures, that may be of the same or different material composition, in the barrel section. The tubular springs in these multi-wall bats are typically either in contact with one another, such that they form friction joints, are bonded to one another with weld or bonding adhesive, or are separated from one another forming frictionless joints. If the tubular springs are bonded using a structural adhesive, or other structural bonding material, the barrel is essentially a single-wall construction. U.S. Pat. No. 5,364,095, the disclosure of which is herein incorporated by reference, describes a variety of bats having multi-walled barrel constructions.
It is generally desirable to have a bat barrel that is durable, while also exhibiting optimal performance characteristics. Hollow bats typically exhibit a phenomenon known as the “trampoline effect,” which essentially refers to the rebound velocity of a ball leaving the bat barrel as a result of flexing of the barrel wall(s). Thus, it is desirable to construct a ball bat having a high “trampoline effect,” so that the bat may provide a high rebound velocity to a pitched ball upon contact.
The “trampoline effect” is a direct result of the compression and resulting strain recovery of the bat barrel. During this process of barrel compression and decompression, energy is transferred to the ball resulting in an effective coefficient of restitution (COR) of the barrel, which is the ratio of the post impact ball velocity to the incident ball velocity (COR=Vpost impact/Vincident). In other words, the “trampoline effect” of the bat improves as the COR of the bat barrel increases.
Multi-walled bats were developed in an effort to increase the amount of acceptable barrel deflection beyond that which is possible in typical single-wall designs. These multi-walled constructions generally provide added barrel deflection, without increasing stresses beyond the material limits of the barrel materials. Accordingly, multi-wall barrels are typically more efficient at transferring energy back to the ball, and the more flexible property of the multi-wall barrel reduces undesirable deflection and deformation in the ball, which is typically made of highly inefficient material.
An example of a multi-wall ball bat 100 is illustrated in FIG. 1. The barrel 102 of the ball bat 100 includes an inner wall 104 separated from an outer wall 106 by an interface shear control zone 108 or layer, such as an elastomeric layer, a friction joint, a bond-inhibiting layer, or another suitable layer. Each of the inner and outer walls 104, 106 includes one or more plies 110 of one or more fiber-reinforced composite materials. Alternatively, one or both of the inner and outer walls 104, 106 may include a metallic material, such as aluminum. A ball bat having this construction is described in detail U.S. patent application Ser. No. 10/712,251, filed on Nov. 13, 2003, the disclosure of which is hereby incorporated by reference.
One way that a multi-wall bat differs from a single-wall bat is that there is no shear energy transfer through the interface shear control zone(s) (“ISCZ”) in the multi-wall barrel, i.e., through the region(s) between the barrel walls that de-couple the shear interface between those walls. As a result of strain energy equilibrium, this shear energy, which creates shear deformation in a single-wall barrel, is converted into bending energy in a multi-wall barrel. And since bending deformation is more efficient in transferring energy than is shear deformation, the walls of a multi-wall bat typically exhibit a lower strain energy loss than does a single wall design. Thus, multi-wall barrels are generally preferred over single-wall designs for producing efficient bat-ball collision dynamics, or a better “trampoline effect.”
To illustrate, FIG. 2 shows a graphical comparison of the relative performance characteristics of a typical wood bat barrel, a typical single-wall bat barrel, and a typical double-wall bat barrel. As FIG. 2 illustrates, double-wall bats generally perform better along the length of the barrel than do single-wall bats and wood bats. While double-wall bats have generally produced improved results along the barrel length, these results still decrease to an extent as impact occurs away from the barrel's “sweet spot.”
The sweet spot is the impact location in the barrel where the transfer of energy from the bat to the ball is maximal (i.e., where the trampoline effect is greatest), while the transfer of energy to a player's hands is minimal. The sweet spot is generally located at the intersection of the bat's center of percussion (COP), and the first three fundamental nodes of vibration. This location, which is typically about 4 to 8 inches from the free end of the barrel (it is shown at 6 inches from the free end of the barrel in FIG. 2, by way of example), does not move when the bat is vibrating in its first (or fundamental) bending mode. As a result, when a ball impacts the sweet spot, the bat does not vibrate, and none of the initial energy of the ball is lost to the bat. Moreover, a player swinging the bat does not feel vibration when the ball impacts the sweet spot.
The barrel region between the sweet spot and the free end of the barrel, and the barrel region between the sweet spot and the tapered section of the bat, in particular, do not exhibit the optimal performance characteristics that occur at the sweet spot. Indeed, in a typical ball bat, the barrel performance, or trampoline effect, decreases considerably as the impact location moves away from the sweet spot. As a result, a player is required to make very precise contact with a pitched ball to achieve optimum results, which is generally very challenging. Thus, a need exists for a ball bat that exhibits improved performance, or trampoline effect, at barrel regions away from the sweet spot.